1. Field of the Invention
This invention relates to a thermal head, and an electronic equipment, such as a facsimile machine, printer, plotter or bar code printer, having such a thermal head.
2. Description of the Related Art
In recent years, with the spread of printers and facsimile machines of the thermal or thermal transfer type equipped with a thermal head, downsizing and large-scale-integration of the individual parts are on the increase in an effort to meet expanding demands for compact size and low cost. As downsizing and energy-saving have been demanded for electronic equipment, it is important and inevitable to reduce a total energy consumption, energy particularly in battery-operated equipment. To this end, the following conventional methods have been considered:
(1) The glazed layer (thin film) on the substrate, on which heat generating elements of a thermal head are arranged, is formed from an increased thermally insulating organic material instead of glass to retard heat conduction toward a lower part of the substrate. For example, assuming that the glazed layer is a polyimide layer of 3 to 15 .mu.m in thickness, it is possible to reduce essential energy supply for a single heat generating element from 0.25 mJ to about 0.13 mJ.
(2) In another method, as indicated by a curve 21 of FIG. 4, a large amount of power is supplied in a short time (t.sub.0 to t.sub.2), for energy supply to heat generating elements, to increase the peak temperature by making the temperature gradient steep, thus increasing the ratio of energy quantity (area of the shaded region above the color developing temperature) for color development of a recording paper to total energy quantity supplied. According to this method, compared to the case of a relatively long-time (t.sub.0 to t.sub.1) energization at a low peak temperature as indicated by a curve 22, though a total quantity of energy supply (product of the power and the energizing time) is substantially the same, the energy quantity for color development is substantially increased, thus improving the energy efficiency. Therefore, to the contrary, when the same printing result is to be obtained, as indicated by a curve 23 of FIG. 4, it is possible to reduce the total amount of energy supply by further shortening the energizing time (t.sub.0 to t.sub.3) with the same energy quantity (area of the shaded region above the color developing temperature) for color development as the case of the curve 22.
In the above method (2), since it is necessary to supply a large amount of power to the individual heat generating element in a short time, the number of heat generating elements to be energized simultaneously is limited to a small number in view of a current capacity of the power source in use. The reasons for this will now be described.
Generally, in controlling a thermal head, a plurality of driver ICs are used to drive and control the respective heat generating elements. For example, in the thermal head 11 shown in FIG. 5, 27 driver ICs 14-1 to 14-27 are mounted on a thermal head substrate 12, and each driver IC drives 64 heat generating elements so that 27 driver ICs drive a row 13 of 1728 heat generating elements in total. These 27 driver ICs are divided into four blocks including 7 driver ICs, 7 driver ICs, 7 driver ICs and 6 driver ICs, and each block is energized and controlled, in time sharing mode, by the strobe signals STR1 to STR4 supplied from an external source. Namely, a block is selected for every strobe signal so that the corresponding 64.times.7 (or 64.times.6) heat generating elements are energized. For example, as shown in FIG. 6, assuming that the pulse width of each strobe signal STR1 to STR4 is 2 ms, four blocks of driver ICs are successively driven so that printing for a single line (1728 heat generatings elements) takes place in 8 ms in total.
Now assuming that, for example, a resistance R of every heat generating element is 3000 .OMEGA. and a driving power V is 24 V, a current I.sub.e flowing in each heat generating element and a consumption power P.sub.e of the heat generating element are expressed by the following equations (1) and (2): EQU I.sub.e =V/R=8 [mA] (1) EQU P.sub.e =VXI=0.192 [W] (2)
Therefore, in the total heat generating elements energized and driven concurrently with the individual strobe signals, a power P.sub.max at maximum expressed by the following equation (3) will be consumed: EQU P.sub.max =0.192 [W]X64.times.7.perspectiveto.86 [W] (3)
An energy consumption E for each heat generating element is expressed by the following equation (4): EQU E.sub.e =0.192 [W].times.2[ms]=0.384 [mJ] (4)
In this case, if the peak of heat generating temperature of each heat generating element is to be increased, a resistance of the heat generating element is lowered to allow a large amount of current to flow. However, this requires a power source having a large current capacity in total as the power consumption of the individual heat generating element increases. Consequently downsizing and energy saving of the electronic equipment are difficult to achieve.
Consequently it is essential to reduce the number of heat generating elements to be energized concurrently with the individual strobe signals to prevent any increase of the maximum power consumption. For this purpose, the driver ICs should be divided into many blocks to minimize the number of driver ICs for each block. The number of strobe signals also should therefore be increased, depending on the number of blocks.
However, if the number of strobe signals was merely increased, it would not be compatible with the conventional type thermal head. Also the wiring due to the increased number of signal lines would become complicated, thus resulting in an increased cost of production.